A leveling valve of this kind is known from DE-PS 33 33 888. In a valve housing, a spring-loaded double valve body is provided, possessing an inlet valve with an inlet seat for a conduit leading to one or several pneumatic bellows. A hollow valve plunger which is supported in a movable way a with its upper end forms an outlet seat assigned to the double valve body acts jointly with the double valve body. The valve plunger is supported by means of a supporting element at a shaft accommodated at the valve housing in a rotating way, that is at a cam formed at this shaft. The shaft may be rotated by means of an actuating lever at the outside of the valve housing so that in this way a rotating motion of the shaft is transformed into an axial motion of the valve plunger. In order to adjust a locking position between inlet valve and outlet valve the valve plunger is supported in a rotating way in relation to the supporting element in a thread linking both parts with on another, the supporting member in turn being guided in the valve housing in a torsion-proof and axially movable manner. A gasket for the sealing of the conduit leading to the pneumatic bellows is assigned to the valve plunger, which is disposed in an only radially open groove in a special inset. The individual parts of the leveling valve are mostly designed as metal parts and are manufactured in an accordingly precise manner, usually by means of a machining process. The bearing of the rotatable shaft is located on one side of a central axis constituted by the valve plunger, the double valve body and the supporting element. This bearing is bedded in a sprue spigot in the valve housing made of metal, causing the valve housing to be designed in the necessary costly manner.
In DE-OS 2451349 a leveling valve is shown whose shaft that can be rotated by means of the actuating lever is received in a continuous bore hole in the valve housing, the bearing being disposed symmetrically to the central axis of the valve plunger. In this way, the support of the supporting element and thus of the valve plunger on the cam of the shaft is improved. Between the double valve body and the cam of the shaft, the valve plunger and the support element are provided both of which may be screwed onto one another by means of threads in order to secure a membrane that serves the purpose of sealing the conduit leading to the pneumatic bellows. However, it cannot be used for adjusting a locking position. In the case of this leveling device some structural elements are already made of synthetic material, e.g. the valve body and the supporting element. It is nothing special to produce valve bodies out of synthetic material or an elastomer, in order to exploit the flexibility of this material in connection with valve seats. The good sliding characteristics of synthetic material bring about a reduction of the friction between supporting element and shaft or cam by means of the supporting element. A design fundamentally different from the design made of metal aiming at designing as many essential structural elements of a leveling valve as synthetic material parts cannot be recognized.
All known leveling valves possess a number of structural elements which to a large degree consist of metal and require careful processing in regard to tolerances in order to achieve the desired operating safety. As a rule, the valve housing which is also made of metal, is designed in several parts in order to enable or simplify the assembly of the leveling valve. The bedding of the rotatable shaft disposed on one side of the central axis of the leveling valve formed by the valve plunger must be arranged with the necessary care so that the take-up of forces of a spring holding the supporting element in contact with the cam of the shaft is rendered possible. In addition, such a leveling valve must be of a certain size, on the one hand, in order to offer the required sections for the ventilation of the pneumatic bellows; on the other hand, one seeks to offer this kind of leveling valves as inexpensively as possible.